MSC4040: Update SRV service name to IANA registration

proposals/4040-update-srv-service-name.md

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+# MSC4040: Update SRV service name to IANA registration
+
+The Internet Assigned Numbers Authority (IANA) holds a registry of all service names and ports in use
+by Internet applications. Unfortunately, the service name `matrix` was registered an eternity ago for
+use in an unrelated project, which makes our usage of it improper at best. On August 4, 2023 our
+registration of port 8448 was accepted, with an included service name of `matrix-fed`. This MSC
+proposes a backwards-compatible change to use this newly registered service name.
+
+The registrations with keyword "matrix" can be found [here](https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml?search=matrix).
+
+This fixes https://github.com/matrix-org/matrix-spec/issues/400
+
+See also https://github.com/matrix-org/matrix-spec/issues/1576
+
+## Proposal
+
+The [current specification](https://spec.matrix.org/v1.7/server-server-api/#resolving-server-names) for
+resolving server names requires an implementation to look up `_matrix._tcp` SRV records in some steps.
+Those steps become *deprecated* and new steps which use `_matrix-fed._tcp` are added immediately before
+each existing step.
+
+Deprecation in context of this MSC is to discourage continued use, and to queue the affected steps for eventual
+removal from the specification under the [deprecation policy](https://spec.matrix.org/v1.7/#deprecation-policy).
+
+The new steps thus become:
+
+1. (unchanged) If the hostname is an IP literal, then that IP address should be used, together with the
+   given port number, or 8448 if no port is given. The target server must present a valid certificate for
+   the IP address. The `Host` header in the request should be set to the server name, including the port if
+   the server name included one.
+
+2. (unchanged) If the hostname is not an IP literal, and the server name includes an explicit port, resolve
+   the hostname to an IP address using CNAME, AAAA or A records. Requests are made to the resolved IP address
+   and given port with a `Host` header of the original server name (with port). The target server must present
+   a valid certificate for the hostname.
+
+3. (steps added/deprecated) If the hostname is not an IP literal, a regular HTTPS request is made to `https://<hostname>/.well-known/matrix/server`,
+   expecting the schema defined later in this section. 30x redirects should be followed, however redirection
+   loops should be avoided. Responses (successful or otherwise) to the `/.well-known` endpoint should be
+   cached by the requesting server. Servers should respect the cache control headers present on the response,
+   or use a sensible default when headers are not present. The recommended sensible default is 24 hours. Servers
+   should additionally impose a maximum cache time for responses: 48 hours is recommended. Errors are recommended
+   to be cached for up to an hour, and servers are encouraged to exponentially back off for repeated failures.
+   The schema of the `/.well-known` request is later in this section. If the response is invalid (bad JSON,
+   missing properties, non-200 response, etc), skip to step 4. If the response is valid, the `m.server`
+   property is parsed as `<delegated_hostname>[:<delegated_port>]` and processed as follows:
+
+   1. (unchanged) If `<delegated_hostname>` is an IP literal, then that IP address should be used together
+      with the `<delegated_port>` or 8448 if no port is provided. The target server must present a valid TLS
+      certificate for the IP address. Requests must be made with a `Host` header containing the IP address,
+      including the port if one was provided.
+
+   2. (unchanged) If `<delegated_hostname>` is not an IP literal, and `<delegated_port>` is present, an IP
+      address is discovered by looking up CNAME, AAAA or A records for `<delegated_hostname>`. The resulting IP
+      address is used, alongside the `<delegated_port>`. Requests must be made with a `Host` header of
+      `<delegated_hostname>:<delegated_port>`. The target server must present a valid certificate for `<delegated_hostname>`.
+
+   3. **(ADDED)** If `<delegated_hostname>` is not an IP literal and no `<delegated_port>` is present, an
+      SRV record is looked up for `_matrix-fed._tcp.<delegated_hostname>`. This may result in another hostname
+      (to be resolved using AAAA or A records) and port. Requests should be made to the resolved IP address and
+      port with a `Host` header containing the `<delegated_hostname>`. The target server must present a valid
+      certificate for `<delegated_hostname>`.
+
+   4. **(DEPRECATED)** If `<delegated_hostname>` is not an IP literal and no `<delegated_port>` is present, an
+      SRV record is looked up for `_matrix._tcp.<delegated_hostname>`. This may result in another hostname (to
+      be resolved using AAAA or A records) and port. Requests should be made to the resolved IP address and port
+      with a `Host` header containing the `<delegated_hostname>`. The target server must present a valid certificate
+      for `<delegated_hostname>`.
+
+   5. (unchanged) If no SRV record is found, an IP address is resolved using CNAME, AAAA or A records. Requests
+      are then made to the resolve IP address and a port of 8448, using a `Host` header of `<delegated_hostname>`.
+      The target server must present a valid certificate for `<delegated_hostname>`.
+
+4. **(ADDED)** If the `/.well-known` request resulted in an error response, a server is found by resolving
+   an SRV record for `_matrix-fed._tcp.<hostname>`. This may result in a hostname (to be resolved using AAAA or A
+   records) and port. Requests are made to the resolved IP address and port, with a `Host` header of `<hostname>`.
+   The target server must present a valid certificate for `<hostname>`.
+
+5. **(DEPRECATED)** If the `/.well-known` request resulted in an error response, a server is found by resolving
+   an SRV record for `_matrix._tcp.<hostname>`. This may result in a hostname (to be resolved using AAAA or A
+   records) and port. Requests are made to the resolved IP address and port, with a `Host` header of `<hostname>`.
+   The target server must present a valid certificate for `<hostname>`.
+
+6. (unchanged) If the `/.well-known` request returned an error response, and the SRV record was not found, an IP
+   address is resolved using CNAME, AAAA and A records. Requests are made to the resolved IP address using port
+   8448 and a `Host` header containing the `<hostname>`. The target server must present a valid certificate for
+   `<hostname>`.
+
+## Potential issues
+
+Us using `matrix` as a service name has been an issue all along. This MSC aligns the specification with our
+formal IANA registration.
+
+## Alternatives
+
+We could modify our IANA registration further to pick a different name, however the service name needs to be
+descriptive and short. The author notes that `matrix-server` was attempted in registration, which was declined
+during Expert Review of the application.
+
+`matrix` cannot be taken over via IANA process. It was assigned before dates were put on records in 2000.
+
+Non-options for names include `mxfed`, `matrixf`, and similar.
+
+A potential way forward is to remove SRV resolution entirely, however prior effort via
+[MSC3922](https://github.com/matrix-org/matrix-spec-proposals/pull/3922) are expected to be *rejected* instead
+(as of writing, August 4th, 2023).
+
+## Security considerations
+
+There is increased DNS load with this change, due to having to potentially look up 6+ records in the worst case
+scenario. DNS servers should be more than capable of caching, and handling such a spike of requests.
+
+## Unstable prefix
+
+None applicable.
+
+## Dependencies
+
+None applicable.